I use styrafoam packing sheets in-between pans when I stack them, and I condition with solid Crisco. I've seen a crescent-shaped shiny spot develop across from the handle on the underside of the pan if I stack them iron-on-iron without spacers. I try to go through the stacks semi-regularly to look for rust spots, etc.
U-haul stores sell rolls of perforated styrafoam packing sheets that are a good size for pans up to size 8. For larger pans, I just use a sheet-and-a-half or two sheets to cover the distance.